1. Field of the Invention
This invention relates to a capacitor-switched filter stabilizing circuit, and in particular to a floating node corrected, integrated switched-capacitor frequency-dependent negative resistance (FDNR) filter. Accordingly, it is a general object of this invention to provide new and improved circuits and filters of such character.
2. General Background
A conventional low-pass filter includes serial inductances and shunt capacitances, together with an input resistance and a load resistance.
By dividing the values of the various reactances of the elements by j.omega., an inductance is replaced by a resistor, a capacitor is replaced by an active circuit, and a resistor is replaced by a capacitor. The advantage of such a circuit, consisting of resistors, capacitors, and operational amplifiers, is that it contains no inductors and can therefore be integrated on an LSI (large-scale-integration) chip. However, the required resistor values of several megohms make integration inefficient.
The inefficiency of high resistance values can be overcome by substituting switched capacitors for resistors. Normally, a capacitor can not conduct direct current. However, by repetitively reversing the plates of each capacitor across a direct current source, the charges on the plates repeatedly change polarity, thereby resulting in an average dc current through the capacitor. This type of filter technique is described in detail in "Switched-Capacitor Filter Design Using the Bilinear Z-Transform", Gabor C. Temes, et al., IEEE Transactions on Circuits and Systems, December, 1978, pp. 1039-1043.
Switched capacitor filters are of special interest because high precision RC filters can be provided on integrated circuit chips by using MOS switched capacitor technology. Using this technique, the RC values become a function of capacitor ratios, which can be very accurately controlled.
Switched capacitor filters that employ a frequency-dependent negative resistance (FDNR) closely simulate LC filters, thereby retaining their pass-band insensitivity. They can be made tunable and can be designed directly from LC filter tables.
The repetitive operation of a pair of serially connected switched-capacitor resistors tends to cause a charge to accumulate at their junction, creating a "floating node", problem. This accumulated charge alternates between the two switched capacitors, as they are switched, causing a square wave at the switching frequency to appear at their junction when the voltage across the capacitors is zero. When the voltage across the capacitors is not zero, it adds to the square wave, causing associated amplifiers to clip the composite signal, thereby disrupting operation.